1. Field of the Invention
The invention relates to exposure control apparatus for use in a camera, and in particular to exposure control apparatus for providing an exposure program related to scene light intensity in which the size of an exposure aperture is proportional to exposure time.
2. Description of the Prior Art
When taking photographs there are two competing, and often times conflicting, parameters to be considered. These two parameters are the shutter speed, or exposure time, and the camera lens aperture size or f-number. Maximum depth of field is obtained with the smallest aperture size (highest f-number); and fast shutter speeds (short exposure times) are capable of "freezing" moving objects in the scene to be photographed. While there are some applications where a camera operator might want an extremely small depth of field or a blurred image indicative of motion, generally the camera operator prefers both a large depth of field and an ability to "freeze" motion. But unless the camera operator is very fortunate to be photographing an unusually bright scene, he must sacrifice to some extent the depth of field or the shutter speed, or both, in order to receive a sufficient amount of light to expose the film properly. Whether to increase the aperture size (decrease the f-number) or slow the shutter speed (increase the exposure time) or do both is a choice a professional photographer usually makes based upon his experience and knowledge of the art. On the other hand, there is a large class of photographers who do not want to be troubled with making the choice between shutter speed and aperture size and who prefer simply to press a shutter release button and take a picture. For these photographers, it is desirable to incorporate into the camera some means for automatically setting both the shutter speed and the aperture size in response to scene illumination. Further, the relationship between aperture size and shutter speed, i.e., the shutter program, should in many situations be a compromise between aperture size and shutter speed-not a sacrifice of one for the other.
Exposure control apparatus is known in the art that provides an exposure program in which the size of an exposure aperture is proportional to an exposure interval, e.g. the larger the aperture size the longer the exposure interval and the smaller the aperture the shorter the exposure time, to produce an optimum compromise between both exposure parameters. In particular, apparatus is known for "compromising" aperture size and shutter speed and, in doing so, the exposure aperture is established prior to shutter opening movement. For example, U.S. Pat. No. 3,893,134, assigned to the assignee of the present invention, discloses exposure control apparatus comprising a shutter and a photoconductively controlled diaphragm which forms a variable exposure aperture between a minimum and a maximum size. The diaphragm is latched to set the exposure aperture with the size thereof being determined by the light level sensed by the photoconductive element. As the exposure aperture is set, a movable control member is synchronously actuated. At a later time which is related to scene light intensity, the control member releases the shutter to uncover the exposure aperture. At a still later time, a closing member is actuated to permit the shutter to move to cover the exposure aperture, thereby terminating the exposure interval. The time interval between when the control member releases the shutter to begin the exposure interval and when the closing member is released to terminate the exposure interval is directly proportional to exposure aperture size. Thus, a large aperture size indicating a low level of light intensity results in a large exposure interval, and a small aperture size indicating a high level of light intensity results in a small exposure interval.
The exposure program disclosed in the aforementioned U.S. Pat. No. 3,893,134 suffers from a disadvantage in that under all scene light conditions, except extreme low light conditions, a time delay exists between when the diaphragm is set and when the shutter is opened. This delay prolongs the period between when the camera is actuated and when an exposure is terminated as well as increases the possibility that an exposure will be effected under scene light conditions different from the conditions under which the diaphragm was set and the shutter was intended to be operated. Furthermore, the overall apparatus disclosed therein is relatively complicated in that it involves several moving and interdependent elements which complicate assembly operations and affect overall production cost. The attractiveness of an exposure control mechanism is related to the degree it is self-acting, its simplicity of design and cost of manufacture. Thus, it is desirable to have an automatic exposure control apparatus capable of achieving the above discussed exposure program while not sacrificing simplicity and cost.